1. Field of the Invention
The present invention relates to the leather tanning arts. More specifically, the present invention relates to a long nip press for drying tanned leather hides.
2. Description of the Prior Art
Leather tanning is the process of converting raw hides or skins into leather. Hides and skins have the ability to absorb tannic acid and other chemicals that prevent them from decaying. FIG. 1 is a general flow diagram of the leather tanning and finishing process. The raw hides are “cured,” a process which involves salting and/or drying the hide once its been stripped from the animal.
The first steps, commonly referred to as the “beamhouse” operations 10, prepare the hides for tanning 20. The cured hides are trimmed and soaked to remove salt and other solids, and to restore moisture lost during curing. The hides are then fleshed to remove excess tissue and impart a uniform thickness. The hair is removed from the hides by soaking in a lime/water mixture to loosen the hairs and then mechanically removing the loosened hairs.
These prepared hides are now ready for the tanning operations 20. Tanning may be performed using either trivalent chromium salts or vegetable tannins extracted from specific tree barks. Chrome tanned leather is softer, more pliable, and quicker to produce than vegetable tanned leather. Chrome tanning is performed using a one-bath process that is based on the reaction between the hide and the chromium salt.
Following chrome tanning, the tanned leather is wrung (or sammied) to dry the hide. This process of removing excess water from a hide is known as dewatering. The tanning liquors and water baths used on the hides in the tanning process saturate the hide with moisture. The wringing process reduces this water content to about 55% and can be achieved by a variety of machines. Wringing machines typically consist of two large rolls, which squeeze excess from the hide. Other common machines use a large mangle with felt covered rollers to press the hide.
After wringing, the tanning process may be repeated and/or dyes may be applied. The tanned hide is then oiled (i.e. fat liquoring) to replace natural oils lost during the tanning process. The leather is dried again, to a 10-20% water content, by one of several methods (air drying, drying in a toggling or pasting unit, vacuum drying, or high-frequency drying) and is ready for finishing.
Finishing processes 30 include conditioning, staking, dry milling, buffing, spray finishing, and plating. Leathers may be finished in a variety of ways including: buffing to produce a suede finish; lacquering to produce a glossy patent leather finish; and waxing, shellacking, or treating with pigments, dyes and resins to achieve a smooth colored finish.
As mentioned, the leather industry uses large presses to wring excess water from the hides after the tanning operation is complete. These machines typically use large rubber covered squeeze rolls juxtaposed in close proximity on another. Two felt belts pass between the squeeze rolls with a wet hide sandwiched in between. FIG. 2 is a side cross-sectional diagram of a conventional leather press having a center top roll 200 in contact at two short pressure points 240 (or nips) with two lower rolls 210. The top roll and the bottom rolls are contained within the rotating felt belts 220 and 230 respectively. The hides are fed between the rolls by the felt belts and excess water is squeezed out at the nips.
The more rolls that are in a press; the more press nips can be formed and the more efficient the press is in removing water from the hides. For example, one roll over two rolls produces two press nips (as shown in FIG. 2), whereas two rolls over three rolls produces four press nips. Another aspect of dewatering the hide is the width of the nip. The larger the roll diameter the wider the nip. The hardness of the roll cover also plays a part in the nip width. The softer the cover the wider the nip. Thus, machine size is a function of the number of rolls, the size of the rolls, and the roll cover material. However, more rolls typically means a higher cost machine.
Regardless of the number of rolls, the leather press must precisely control the pressure applied to the hides. Too much pressure on a saturated hide can rupture the grained (flowered) side as water is intended to exit the hide in only one direction, which is to the flesh side.
Further, the dewatering process is dependent on the efficiency of the belts in carrying the expelled water away from the hide. Hence, the felt belts must be able to handle the amount of water being pressed from the leather hide during the wringing process.
Therefore, a need exists for a wringing/press device for dewatering hides in the leather industry that is compact and cost efficient yet has a high dewatering efficiency.